1. Field of the Invention
This invention relates generally to infrared (IR) scanners and more specifically to multifaceted scanning wheels for such scanners.
2. Description of Related Art
Infrared scanners, to which the present invention relates, comprise a multifaceted scanwheel mounted for rotation on a bearing, means for projecting infrared light at the rim of the wheel as it spins at high speed and for detecting the light reflected by the facets, and a drive motor and associated electronics for spinning the scanwheel at a controlled, steady speed.
To meet minimum optical performance requirements, the scanwheel facets must be precisely oriented relative to the axis of rotation of the scanwheel and, in order to minimize power consumption, the friction encountered by the bearing should be as low as possible. When used in military applications, such scanners may be subjected to shock loads up to 100 G and also to high frequency vibration and, hence, must be strong and mechanically stiff, with a resonant frequency above a certain level, such as 500 Hz. Such scanners are also required to work under extremes of temperature, which may range from -55.degree. to +72.degree. C.
Minimum weight is frequently an additional design requirement. Hence, the bearing is usually made of steel for strength, the scanwheel of aluminum for minimum weight. These metals have radically different coefficients of thermal expansion and of conductivity. As a result, when the scanwheel assembly is exposed to expected changes in temperature, and particularly to a drop in temperature, stresses are introduced which can result in compression of the bearing, causing dramatically increased power consumption. And a shift in temperature either up or down over the extremes which the scanner may be expected to encounter may cause drastic distortion of the scanner wheel facets, unacceptably degrading optical performance. These stresses are inherent in the construction of scanwheels which typically include a facet ring containing a plurality of flat, high-reflective facets, a hub at the center of the ring for mounting it on a bearing assembly, and a disk-shaped web connecting the ring to the hub. The hub is tightly fitted onto the bearing assembly while the scanning wheel is at an elevated temperature and then cooled to achieve an interference fit. As the scanwheel cools, it contracts, placing its web in compression. Unless the compression is relieved, it exerts a reaction force against the bearing and the facet ring, distorting both.
Measures to isolate the bearing and facets from thermally induced stresses tend to conflict with the need for shock and vibration resistance, because stress isolation calls for increased flexibility, whereas resistance to shock and vibration call for increased stiffness.